Chain tensioning devices are used to control power transmission chains as the chain travels between a set of sprockets. Such chains usually have at least two separate strands, spans or lengths extending between the drive sprocket, such as a crankshaft sprocket, and the driven sprocket, such as a cam sprocket. The strand between the sprockets where the chain leaves the driven sprocket and enters the drive sprocket is frequently under tension as a result of the force imposed on the chain by the drive sprocket. The strand between the sprockets where the chain leaves the drive sprocket and enters the driven sprocket is frequently under reduced drive tension or slack due to the absence of driving force exerted on that strand. In systems with large center distances between the sprockets, both strands may evidence slack between the sprockets.
As a consequence, it is essential to the proper operation of the chain and sprocket system that a proper degree of engagement between the chain members and the sprockets is maintained during operation of the system. One aspect of maintaining such engagement of chain and sprocket is maintaining a proper degree of tension in the chain strands. The loss of chain tension can cause undesirable vibration and noise in the chain strands. The loss of chain tension also increases the possibility of chain slippage or unmeshing from the teeth of the sprocket, reducing engine efficiency and, in some instances, causing system failures. For example, it is especially important to prevent the chain from slipping in the case of a chain-driven camshaft in an internal combustion engine because misalignment of camshaft timing by several degrees can render the engine inoperative or cause damage to the engine.
The tension of the chain can vary due to wide variations in temperature and linear expansions among the various parts of an engine. Moreover, wear to the chain components during prolonged use also may produce a decrease in the chain tension. In addition, the intermittent stress placed on the chain devices in automotive applications due to variation in engine speed, engine load and other stress inducing occurrences can cause temporary and permanent chain tension.
To maintain tension in such transmission systems, tensioner devices have been used to push a tensioner arm against the chain along a chain strand. Such transmission systems typically press on the chain mechanically deflect the strand path imparting under the desired degree of tension on the chain. Current tensioner devices for performing this function, such as torsion spring tensioners, utilize the energy stored in a wound spring to drive the tensioner arm, such as shown in Ojima, U.S. Pat. No. 5,030,170. The small size of torsion spring tensioners makes them highly suitable in many situations. However, they often require an excessive spring load to effectively dampen chain vibrations and maintain a constant spring tension.
Hydraulic tensioner devices typically have a plunger slidably fitted into a chamber and biased outward by a spring to provide tension to the chain. Hydraulic pressure from an external source, such as an oil pump or the like, flows into the chamber through a check valve and passages formed in the housing of the device. The plunger may move outward against the chain, directly against a tensioner arm principally by an internal spring or similar structure and the plunger position is maintained in large part by hydraulic pressure within the housing. Such a hydraulic tensioner as used with a tensioner arm or shoe is shown in Simpson et al., U.S. Pat. No. 5,967,921.
Hydraulic tensioners frequently are preferred over torsion spring tensioners because they are much better at dampening chain movement and maintaining constant chain tension. For example, as a chain traverses its path, it may vibrate or “kick” causing the chain to push against the tensioner arm. The force of the kick is transferred to the tensioner device causing the hydraulic plunger to move in a reverse direction away from the chain. This reverse movement is resisted by the hydraulic fluid in the chamber, as flow of the fluid out of the chamber is restricted by the check valve assembly. In this fashion, the tensioner achieves a so-called no-return function, i.e., movements of the plunger are relatively easy in one direction (towards the chain) but difficult in the reverse direction. In addition, rack and ratchet assemblies also may be employed to provide a mechanical no-return function.
In some applications, however, the size and bulk of hydraulic tensioners can present difficulties in mounting and operating such tensioners where the available space, is better suited for torsion spring tensioners. To overcome the difficulty created by the size of hydraulic tensioners, lever systems have been employed that allow the mounting of the hydraulic tensioner at a distance from the chain assembly. Through the lever system, the hydraulic tensioner imparts pressure on one or more strands of the chain assembly thereby maintaining chain tension.
However, such lever mechanisms add to the complexity of the tensioner system and involve additional moving parts with a concomitant increase in maintenance expenses, problems and equipment failures. The use of such pivoted lever mechanisms may also diminish the ability of the hydraulic tensioners to dampen chain vibration. In addition, the mechanical limitations of the typical rod and piston design of hydraulic tensioners often limit the amount of slack which can be taken up by the tensioner during the life of the chain. One example of such a tensioner device is described in Sato et al., U.S. Pat. No. 5,318,482.